Neem Oil Is Not Killing Spider Mites

Eradicating spider mites takes multiple applications of neem oil.

When the leaves on your plant look like they are covered with thousands of tiny spots, chances are you have a spider mite infestation. If you're an eco-conscious home gardener, you'll probably reach for a plant-based pesticide such as neem oil to minimize the effects of chemicals on the environment. If the neem oil is not effective, you might be using it incorrectly.

No Ordinary Pest

Spider mites are not insects, although they are sometimes categorized as insect pests. Rather -- as their name implies -- they are related to spiders and classified as arachnids. Tiny ones, however: They are less than 1/20th of an inch in length and look like moving black dots to the naked eye, according to the University of California Statewide Integrated Pest Management Program. Although you don't want to spray spider killer on your plants, the fact that they are arachnids means that they react to some pesticides in ways different from other common insect pests -- such as aphids and scale -- might react.

Resistant to Pesticides

Spider mites can be difficult to eradicate with pesticides, especially if populations are large. They have many natural enemies that usually keep their populations down in the garden, but indoors those predators are not around to gobble them up. In addition, pesticide use in the garden can kill those beneficial predators, leaving the spider mites to thrive and proliferate. Pesticides containing carbaryl actually increase spider mite reproduction, according to the University of California Statewide Integrated Pest Management Program. Neem oil, on the other hand, can be effective against spider mites, but only if it is used properly.

Neem Oil Effectiveness

The most effective way to use neem oil to eradicate spider mites is to apply it once and then release natural predators into the area, but this isn't always practical or feasible -- especially if the mites are infesting indoor plants. Otherwise, stick with the neem oil -- making sure you are applying it correctly -- and combine that with other control methods, keeping in mind that many plants can tolerate a small population of spider mites. If you see a few of them hanging around your woody plant, don't worry too much about it.

Applying Neem Oil

To apply a premixed neem oil correctly, thoroughly wet the plant, agitating the sprayer constantly and paying particular attention to the undersides of the leaves. Reapply the neem oil once a week at the minimum. Because spider mites are arthropods -- they have external skeletons -- you might have to apply it even more often than that. So in short, if the neem oil is not killing your spider mites, it might be because you aren't applying it often enough or maybe you are applying it at the wrong time. Never apply neem oil when temperatures are above 90 degrees Fahrenheit or when the plants are water stressed, meaning they need water or are going through a period of drought.

Other Control Methods

Spider mites like dusty leaves, so if you have houseplants, keep the leaves clean. Spraying the undersides of the leaves with water once a day can discourage spider mites from clustering there. Keep plants well watered if they prefer moist soil, as water-stressed plants are more prone to suffer from spider mite infestations. If you start to see a few mites on your plant, blast them off with a strong stream of water.